The present invention relates to resonant oscillation suppression systems for offshore floating platforms.
Tension Leg Platforms (TLPs) are floating platforms that are held in place in the ocean by means of vertical structural mooring elements called tendons, which are typically fabricated from high strength, high quality steel tubulars, and include articulated connections on the top and bottom (tendon connectors) that reduce bending moments and stresses in the tendon system. Many factors must be taken into account during the design of the tendon system to keep the TLP safely in place including: (a) limitation of stresses developed in the tendons during extreme storm events and while the TLP is operating in damaged conditions; (b) avoidance of any slackening of tendons and subsequent snap loading or disconnect of tendons as wave troughs and crests pass the TLP hull; (c) allowance for fatigue damage which occurs as a result of the stress cycles in the tendons system throughout its service life; (d) limit natural resonance (heave, pitch, roll) motions of the TLP to ensure adequate functional support for personnel, equipment, and risers; and (e) vibrations in the platform system arising from vortex-induced vibrations.
As water depth increases beyond about 4,000 ft, the TLP system cost begins to be driven by the cost of the tendon system due to the length and wall thickness of tendons and by fatigue considerations. To provide adequate platform motion control and to limit the amount of fatigue damage caused by each stress cycle, it has been thought necessary to limit the natural resonance periods of the TLP system (heave, pitch and roll) to the 3-4 second range by increasing the cross-sectional area of the tendon (i.e., by stiffening the “spring” since the “mass” of the platform is set mainly by operational considerations). The increasing requirement for more steel cross-sectional area in addition to length in deeper water causes the tendon system to become heavier, thus increasing the tendon cost and reducing the payload carrying capacity of the platform system, i.e. more and more platform buoyancy is ‘consumed’ merely supporting its own mooring system. This combination of increasing tendon length and tendon wall thickness causes the tendon system to dominate total installed cost of the entire TLP system in deepwater installations, i. e. beyond 6000 ft water depth.
It is therefore an object of the present invention to provide a floating platform system including a passive oscillation suppression system that inhibits resonant responses in the platform system leading to better motions for personnel, equipment and riser support, and to lighter and lower cost tendon systems.